Coupling of the galvanic cell and Z-scheme effects of zinc/gallium dual co-catalysts to enhance the photocatalytic conversion of carbon dioxide using water as an electron donor.

Artificial synthesis is an environment friendly photocatalytic strategy to converse carbon dioxide (CO2) into useful chemicals. However, water (H2O) splitting, producing (hydrogen) H2 strongly, is always a competitive reaction to CO2 conversion. Therefore, proper cocatalysts are generally needed to enhance CO2 conversion but suppress H2 production. In this work, zinc/gallium (Zn/Ga) dual co-catalysts consisting of Zn0 and amorphous ZnGa2O4 species were found to selectively produce carbon monoxide (CO) during the photocatalytic conversion of carbon dioxide (CO2) using water (H2O) as an electron donor over photocatalysts such as NaTaO3, Ga2O3, and ZnGa2O4, and in the electrochemical reduction of CO2 over Zn0 electrodes. It is considered that there are two effects associated with the Zn/Ga dual co-catalysts: (1) a galvanic cell effect between Zn0 and amorphous ZnGa2O4, and (2) a Z-scheme effect in NaTaO3/Zn0/amorphous ZnGa2O4. The coupling of these two effects favored the active and selective evolution of CO during the photocatalytic conversion of CO2 by H2O. In the case of Ga2O3 photocatalyst, 480.8 µmol/h of CO was produced with the presence of Zn/Ga dual cocatalysts. Moreover, the Zn/Ga dual cocatalysts universally worked in the electrochemical reduction of CO2. The partial current toward CO2 conversion was increased from 2.6 to 6.6 mA/cm, and the selectivity toward CO was promoted to from 46.4 % to 74.2 %.